Rotor construction for controlled-pole electric machines

ABSTRACT

A method of constructing a rotor for use in a high speed controlled-pole electric machine includes the steps of positioning an inner layer of remagnetizable permanent magnetic material over substantially an entire circumference of a rotor core exterior surface, and positioning a outer layer of remagnetizable permanent magnetic material positioned over substantially an entire circumference defined by said inner layer of remagnetizable permanent magnetic material. The inner layer of permanent magnetic material can have a first set of magnetic properties and the outer layer of permanent magnetic material can have a distinct second set of magnetic properties. The layers of remagnetizable permanent magnetic material can be formed from a plurality of magnetic blocks. The method can include the step of binding the layers of remagnetizable permanent magnetic material to the rotor using high strength wire.

RELATED APPLICATION DATA

[0001] This is a divisional application of Ser. No. 09/303,070, filedApr. 30, 1999 entitled “ROTOR CONSTRUCTION FOR CONTROLLED-POLE ELECTRICMACHINES”.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to a method of constructing of a rotor, inparticular, to the construction of a rotor for a high speedcontrolled-pole electric machine.

[0004] 2. Description of Related Art

[0005] A controlled pole electric machine includes a stator surroundinga rotor, the rotor having a high permeability, low loss rotor corecarrying a surface layer of magnetizable permanent magnetic material.During the rotor's rotation, the surface layer will pass in closeproximity to the stator. In consequence, single-phase alternatingcurrent passing through concentrated windings about the stator willmagnetize the adjacent magnetic layer into a written-pole pattern ofnorth and south magnetic poles. Subsequently, the magnetic poles willreact with the main windings in the stator to produce desired torquecharacteristics.

[0006] Rotors for use in high speed controlled-pole electric machinesoften employ a special layer of remagnetizable permanent magneticmaterial which inimitably contributes to the unique and desirableperformance of controlled-pole electric machines. Typically, thisremagnetizable permanent magnetic material is strontium ferrite.Strontium ferrite is a ceramic and, as such, is friable with a tensilestrength normally less than 5,000 pounds per square inch [PSI]. As aresult of the brittle nature of strontium ferrite, the rotorconstruction for high speed controlled-pole electric machines mustsatisfy several critical criteria.

[0007] First, the layer of remagnetizable permanent magnetic materialmust be maintained in position with a level of restraint adequate tooffset the centripetal force needed to keep the layer of magneticmaterial traveling in a circle. When deployed in a high speedcontrolled-pole electric machine, a rotor can experience centripetalforces exceeding several thousand PSI. Moreover, the traditional use ofbonding adhesives, like high strength epoxies, under conditions wherethe centripetal force exceeds several thousand PSI, transcends the safelimits recommended for such bonding adhesives.

[0008] Second, unlike conventional fixed-pole electric machines withpermanent magnets, the magnetic layer in a high speed controlled-polemachine can experience a rapid temperature increase, particularly duringthe starting phase of the motor. This temperature increase is caused bythe necessary hysteresis loss in the magnetic layer during therepositioning or adjusting of the magnetic poles of the rotor. Inaddition, the temperature rise can be so rapid that the magnetic layercan become much hotter than the core of the laminated electrical sheetsteel residing underneath the magnetic layer. This temperaturedifferential will cause the magnetic layer to expand relative to thesteel core. Consequently, the rotor construction must accommodate thisthermal expansion to avert potentially large shear forces whichotherwise can result between the magnetic layer and the steel core.

[0009] Third, the favorable performance of a controlled-pole electricmachine can depend upon the very low electrical conductivity exhibitedby the rotor's magnetic layer and any supporting structure. Without therequisite low electrical conductivity, eddy currents in conductingelements can repel the magnetic flux provided by the controlled-poleelectric machine's exciter coil. As a result, the eddy currents canlimit the ability of the magnetic flux to penetrate the magnetic layerin a manner expeditious enough to cause the effective control of therotor poles.

SUMMARY OF THE INVENTION

[0010] The present invention includes a rotor for use in a high speedcontrolled-pole electric machine which overcomes the deficiencies in theprior art. A rotor for use in a high speed controlled-pole electricmachine can include a rotor core having an exterior surface; an outerlayer of remagnetizable magnetic material positioned about the rotorcore exterior surface; and an outer layer of high strength wire bindingthe outer layer of remagnetizable magnetic material to the rotor coreexterior surface. Notably, the rotor core can be a laminated steel rotorcore.

[0011] Where used in a smaller controlled-pole electric machine, only asingle layer of remagnetizable magnetic material may be necessary.However, where used in a larger controlled-pole electric machine,including several layers of remagnetizable magnetic material may beadvantageous. In particular, the present invention can include an innerlayer of remagnetizable magnetic material disposed between the outerlayer of remagnetizable magnetic material and the rotor core exteriorsurface. Moreover, the present invention can include an inner layer ofhigh strength wire for binding the inner layer of remagnetizablemagnetic material to the rotor core exterior surface.

[0012] Each layer of remagnetizable magnetic material can furthercomprise a plurality of magnetic blocks positioned adjacent to oneanother. Each magnetic block can have a separation gap between eachadjacent magnetic block. The separation gap can be calculated to allowfor a thermal expansion of each magnetic block relative to the adjacentmagnetic blocks. Typically, the coefficient of thermal expansion of eachmagnetic block is 20×10⁻⁶ parts per degree Fahrenheit. In oneembodiment, the separation gap can be less than 0.002 inches per inch ofmagnetic block length.

[0013] The rotor can also include an adhesive layer disposed on theinward-facing surface of each magnetic block in each layer ofremagnetizable magnetic material. The use of a pliant adhesive in theconstruction of the present invention can improve the thermalconductivity between the layer of remagnetizable magnetic material andthe rotor core. In addition, because the high strength wire provides thenecessary binding force, the adhesive layer need not be chosen for itsbinding characteristics. Rather, the adhesive can be chosen for itsthermal conductivity, pliability and resistance to thermal aging.

[0014] The high strength wire used to bind the inner layer ofremagnetizable magnetic material preferably should exhibit propertiessimilar to common music wire. Music wire typically has a tensilestrength ranging from 225,000 PSI to over 400,000 PSI and has themagnetic properties of a low coercive permanent magnet. The magneticproperties of the inner layer of high strength wire can serve to improvethe magnetic path of the rotor flux by bridging gaps between separatemagnetic blocks in each layer of remagnetizable magnetic material. Inthe preferred embodiment, the coercivity of the inner layer of highstrength wire typically can be in the order of 50 oersteds. In addition,the residual induction can be approximately one tesla, or 10,000 gauss.

[0015] Where a reduction of losses in the outermost layer ofremagnetizable magnetic material is of paramount concern, an outer layerof high strength wire comprised of a non-magnetic high strength wire,for instance, an alloy like stainless steel, would be desirable.Alternatively, a suitable non-magnetic high strength wire can alsoconsist of a high strength fiber, for instance, carbon fiber. Thus, itis an advantage of the present invention that the use of a non-magneticlayer of high strength wire can reduce losses in the outer layer ofremagnetizable magnetic material caused by flux reversal in the outerwire layer, occurring as the section of the outer layer of high strengthwire passes a stator winding slot.

[0016] A method of constructing a rotor for use in a high speedcontrolled-pole electric machine can comprise the steps of: positioninga plurality of magnetic blocks about a rotor core exterior surface, themagnetic blocks forming an inner layer of remagnetizable magneticmaterial; and, binding the inner layer of remagnetizable magneticmaterial to the rotor core exterior surface using a layer of highstrength wire. The method can further comprise the steps of: positioninga plurality of magnetic blocks about the inner layer of remagnetizablematerial, the magnetic blocks forming an outer layer of remagnetizablemagnetic material; and, binding the outer layer of remagnetizablemagnetic material to the rotor core exterior surface using an outerlayer of high strength wire. One skilled in the art, however, willappreciate that when used in a smaller controlled-pole electric machine,only a single layer of remagnetizable magnetic material may benecessary.

[0017] The positioning step can further include: calculating aseparation gap between each adjacent magnetic block to allow for athermal expansion of each magnetic block; and, separating each magneticblock by the calculated distance. In one embodiment, the separation canbe 0.002 inches for each inch of magnetic block length. The method canfurther include the step of applying a pliant adhesive layer on theinward-facing surface of each magnetic block for holding each magneticblock in place prior to the binding step.

[0018] Finally, the binding step can further include: pretensioning astrand of high strength wire to a tenseness greater than a maximumcentripetal force anticipated to be experienced by the layer ofremagnetizable magnetic material when rotating the rotor in a high-speedcontrolled-pole electric machine; securing one end of the high strengthwire to one end of the rotor; and, wrapping the high strength wire aboutthe layer of remagnetizable magnetic material, securing a second end ofthe high strength wire to a second end of the rotor, thereby forming aplurality of parallel strands having a small but distinct space betweeneach parallel strand of the high strength wire.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 illustrates a longitudinal vertical cross-section of a highspeed controlled-pole electric machine containing an embodiment of thepresent invention.

[0020]FIG. 2A illustrates a longitudinal vertical cross-section of anembodiment of the present invention.

[0021]FIG. 2B illustrates an enlarged view of a portion of the preferredembodiment of the present invention as illustrated in FIG. 2A.

[0022]FIG. 3 illustrates an exploded cut-away view of the magnifiedtransverse vertical partial cross-section illustrated in FIG. 5.

[0023]FIG. 4 illustrates an exploded cut-away view of a magnifiedtransverse vertical partial cross-section of a controlled-pole electricmachine utilizing an outer layer of magnetic high strength wire.

[0024]FIG. 5 illustrates a magnified transverse vertical partialcross-section of the present invention.

[0025]FIG. 6A illustrates a top view of the present invention having asingle strand of high strength wire partially wrapped about theoutermost layer of remagnetizable magnetic material.

[0026]FIG. 6B illustrates a top view of the present invention havingthree strands of high strength wire partially wrapped about theoutermost layer of remagnetizable magnetic material.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] The presently preferred embodiment of the inventive arrangementis shown in the drawings, it being understood, however, the inventivearrangements are not limited to the precise arrangement andinstrumentality shown. FIG. 1 shows the rotor 9 employed in itspreferred application, a controlled-pole electric machine 1. Thecontrolled-pole electric machine 1 includes a stator core 2, a winding 3and a rotor 9. The rotor 9 preferably has a rotor shaft 6, a laminatedsteel rotor core 19 surrounding the rotor shaft 6, and two layers ofremagnetizable magnetic material 4 enveloping the laminated steel rotorcore 19. In addition, two rotor end rings 7 preferably are disposed ateach end of the laminated steel rotor core 19 and abut the two layers ofremagnetizable magnetic material 4.

[0028] As shown in FIG. 2A, the laminated steel rotor core 19 caninclude a steel rotor core 5, having an exterior surface 10 and a rotorlamination layer 11 surrounding the steel rotor core 5. Encasing thelaminated steel rotor core 19, the two layers of remagnetizable magneticmaterial 4 can have an inner layer of remagnetizable magnetic material13 positioned on the exterior surface of the rotor lamination layer 11,and an outer layer of remagnetizable magnetic material 12 adjacent tothe inner layer of remagnetizable magnetic material 13. Furthermore, thetwo end rings 7 directly abut the layers of remagnetizable magneticmaterial 4 and the rotor lamination layer of the laminated steel rotorcore 19.

[0029]FIG. 2B illustrates an enlarged view of the two layers ofremagnetizable magnetic material 4 as illustrated in FIG. 2A. In thepreferred embodiment, an inner layer of magnetic high strength wire 15,attached to groove 20 etched in rotor end ring 7, preferably binds theinner layer of remagnetizable magnetic material 13 to the laminatedsteel rotor core 19. Likewise, an outer layer of non-magnetic highstrength wire 14, anchored to groove 21 etched in rotor end ring 7,preferably binds the outer layer of remagnetizable magnetic material 12about the inner layer of magnetic high strength wire 15, securing theouter layer of remagnetizable magnetic material 12 to the laminatedsteel rotor core 19.

[0030] The inner layer of magnetic high strength wire 15 can haveproperties similar to common music wire. In particular, the inner layerof magnetic high strength wire 15 can have a tensile strength rangingfrom 225,000 to over 400,000 PSI, and the inner layer of high strengthwire has the magnetic properties of a low coercive permanent magnet.Typically, the coercivity can be on the order of 50 oersteds and canhave a residual induction of approximately 1 tesla. Moreover, theexpansion coefficient of the high strength wire 15 can be on the orderof 8.5×10⁻⁶ parts per degree Fahrenheit.

[0031] The outer layer of high strength wire 14 preferably is anon-magnetic high strength wire consisting of the alloy stainless steel.It will be appreciated, however, that one skilled in the art couldsubstitute for stainless steel any number of non-magnetic metallicalloys, or non-metallic high strength fibers, for example, carbon fiber.The advantage of using a non-magnetic high strength wire, such as thestainless steel used to form the outer layer of high strength wire 14,is the reduction of losses in this wire layer.

[0032] Turning now to FIG. 3, it is a particular advantage of thepresent invention that the thickness of the inner layer of magnetic highstrength wire 15, due to its magnetic properties, does not increase theeffective “air gap” between the outer layer of remagnetizable magneticmaterial 12 and the inner layer of remagnetizable magnetic material 13which, preferably, need be as small as practical. Typically, in acontrolled-pole electric machine, an exciter coil embedded in a statorwinding slot (for instance, stator winding slot 18 as shown in FIG. 5),when energized, can produce a strong magnetic flux of alternatingpolarity. Notably, the magnetic properties of the inner layer ofmagnetic high strength wire 15 serve to improve the magnetic path of therotor flux 24 presented by the exciter coil of the controlled-polemachine 1 of FIG. 1, by bridging gaps between the outer layer ofremagnetizable magnetic material 12 and the inner layer ofremagnetizable magnetic material 13. In this way, the rotor flux 24 canpenetrate the magnetic layers 4 of FIG. 1 quickly enough to effectivelycontrol the rotor poles.

[0033]FIG. 4 depicts the interface between a stator core 28 and an outerlayer of remagnetizable magnetic material 31 in a controlled-poleelectric machine having a rotor using an outer layer of magnetic highstrength wire. In FIG. 4, one skilled in the art will observe an area offlux reversal 23, occurring immediately below stator winding slot 27 inthe stator core 28. In particular, FIG. 4 typifies the presence ofmagnetic losses caused by the area of flux reversal 23 in the outer wirelayer 29 which results in consequence of the use of an outer layer ofmagnetic high strength wire in lieu of the non-magnetic layer of highstrength wire 14 of FIG. 2B. These losses in the outer wire layer 29 arecaused by the flux reversal in the wire layer 29 relative to the normalrotor flux 30, as the magnetic wire section 29 passes by a statorwinding slot 27.

[0034]FIG. 5 shows a partial cross-section of the region of intersectionbetween the stator core 2 as shown in FIG. 1 and the outer portion ofthe rotor, as shown in FIG. 2B, consisting of an outer layer ofnon-magnetic high strength wire 14, an outer layer of remagnetizablemagnetic material 12, an inner layer of magnetic high strength wire 15,an inner layer of remagnetizable magnetic material 13, and statorlamination 11. Both the outer layer of remagnetizable magnetic material12 and the inner layer of remagnetizable magnetic material 13 preferablyconsist of a plurality of magnetic blocks 16 positioned concentricallyabout the rotor lamination layer 11, each magnetic block 16 standingadjacent to one another. From FIG. 5, it will be apparent to one skilledin the art that the individual magnetic blocks 16 of both the outerlayer of remagnetizable magnetic material 12 and the inner layer ofremagnetizable magnetic material 13 are positioned such that there is avery small, but distinct, separation gap 17 between each magnetic block16.

[0035] From FIG. 5, it will be apparent that the separation gap 17between each magnetic block 16 widens from the narrowest gap 32 near thebase of each magnetic block 16 to the broadest gap 33 near the surfaceof each magnetic block 16. The amount of the separation 17 preferably iscalculated to allow room for the independent thermal expansion of eachmagnetic block 16 relative to the laminated steel rotor core 19 andneighboring magnetic blocks 16 at the narrowest portion of theseparation 17. Typically, the expansion of a single magnetic block 16 isless than 0.002 inches per inch of magnetic block 16 length. Thus, eachmagnet block 16 should require less than 0.002 inches of space betweeneach of its sides and adjacent magnetic blocks 16.

[0036] An adhesive layer preferably is disposed on the inward-facingsurface of each magnetic block 16. The use of a pliant adhesive in theconstruction of the present invention can improve the thermalconductivity between the magnetic blocks 16 and the laminated steelrotor core 19. In addition, because both the outer layer of highstrength wire 14, and the inner layer of high strength wire 15 providethe binding force necessary to resist the centripetal force experiencedby the magnetic blocks 16 during the operation of the rotor 9, theadhesive layer need not be chosen for its binding characteristics.Rather, the adhesive layer can be chosen for its thermal conductivity,pliability and resistance to thermal aging.

[0037]FIG. 6A is an illustration of a preferred method for applying theouter layer of high strength wire 14 to the outer layer ofremagnetizable magnetic material 12, (and correspondingly, the innerlayer of high strength wire 15 to the inner layer of remagnetizablemagnetic material 13). Initially, the magnetic blocks 16 can bepositioned to form the outer layer of remagnetizable magnetic material12. Subsequently, the outer layer of high strength wire 14 can beattached to the groove 21 etched into end ring 7 by swaging, welding, orby any other suitable method. Likewise, where the use of multiplestrands of high strength wire becomes preferable as in FIG. 6B, eachstrand of high strength wire 14A can be attached to grooves 21A, etchedequidistantly about the end ring 7A circumference. Notably, the use ofmultiple strands of high strength wire 14A may become preferable where adecrease in production time is of paramount concern.

[0038] Prior to wrapping the outer layer of magnetic material with theouter layer of high strength wire 14, the outer layer of high strengthwire 14 preferably is pre-tensioned to a level that is greater than thecentripetal force experienced by the outer layer of remagnetizablemagnetic material 12 during the high speed rotation of the rotor 9.Notwithstanding, the outer layer of high strength wire 14 preferably ispre-tensioned to a tension less than the tensile strength of the outerlayer of high strength wire 14. Specifically, the outer layer of highstrength wire 14 is not pre-tensioned to a level beyond 50% of themaximum tensile strength of the outer layer of high strength wire 14.Where multiple strands of wire are preferred, as in FIG. 6B, each strandof high strength wire 14A should be tensioned individually.

[0039] In either case, it is important for best results that the outerlayer of high strength wire 14 be wrapped about the entire outer layerof remagnetizable magnetic material 12 leaving only a very small, yetdistinct space 25 between each parallel strand of high strength wire 14.This greatly increases the effective electrical resistance of the outerlayer of high strength wire 14, thereby essentially eliminating possibleeddy currents in the outer high strength wire layer 14 which otherwisemay affect the performance of the controlled-pole electric machine 1 ofFIG. 1. For this reason, it is important to have only a single layer ofhigh strength wire 14 at any location. Subsequent to wrapping, the outerlayer of high strength wire 14 is then secured to groove 21 etched intoa second end ring 7 opposite the first end ring 7.

1. A method of constructing a rotor for use in a high speedcontrolled-pole electric machine, comprising the steps of: positioningan inner layer of remagnetizable permanent magnetic material oversubstantially an entire circumference of a rotor core exterior surface;and, positioning an outer layer of remagnetizable permanent magneticmaterial positioned over substantially an entire circumference definedby said inner layer of remagnetizable permanent magnetic material. 2.The method of claim 1, wherein said inner layer of permanent magneticmaterial has a first set of magnetic properties and said outer layer ofpermanent magnetic material have a second set of magnetic properties,said sets of magnetic properties differing from one another.
 3. Themethod of claim 1, wherein at least one of said layers of remagnetizablepermanent magnetic material comprise a plurality of magnetic blocks. 4.The method of claim 1, further comprising the step of binding saidlayers of remagnetizable permanent magnetic material to said rotor coreexterior surface using at least one layer of high strength wire.
 5. Themethod according to claim 4, wherein said at least one layer of highstrength filament comprises an inner layer of high strength filamentbinding said inner layer of remagnetizable permanent magnetic materialto said rotor core exterior surface and an outer layer of high strengthfilament binding said outer layer of remagnetizable permanent magneticmaterial to said inner of remagnetizable permanent magnetic material. 6.The method of claim 1, wherein at least one of said layers ofremagnetizable permanent magnetic material comprise a plurality ofmagnetic blocks, further comprising the step of binding said layers ofremagnetizable magnetic material to said rotor core exterior surfaceusing at least one layer of high strength wire.
 7. A method according toclaim 6, further comprising the step of applying a pliant adhesive layeron an inward-facing surface of said magnetic blocks, said inward-facingsurface relative to said rotor, whereby said plaint adhesive layer canhold each said magnetic block in place prior to said binding step.
 8. Amethod according to claim 3, wherein said positioning step furthercomprises: calculating a separation gap between each adjacent saidmagnetic block to allow for a thermal expansion of each said magneticblock; and, separating each said magnetic block by approximately saidcalculated separation gap.
 9. A method according to claim 8, whereinsaid calculating step further comprises providing approximately a 0.002inch separation gap for each inch of magnetic block length.
 10. Themethod according to claim 4, wherein said binding step furthercomprises: pretensioning a strand of high strength wire to a tensenessgreater than a maximum centripetal force anticipated to be experiencedby said layers of remagnetizable permanent magnetic material whenrotating said rotor in a high-speed controlled-pole electric machine;securing a first end of said high strength wire to a first end of saidrotor; wrapping said high strength wire about said layers ofremagnetizable permanent magnetic material leaving a small but distinctspace between each parallel strand of said high strength wire andentirely covering said layer of remagnetizable permanent magneticmaterial; and, securing a second end of said high strength wire to asecond end of said rotor.
 11. The method according to claim 4, whereinsaid at least one layer of high strength filament comprises an innerlayer of high strength filament binding said inner layer ofremagnetizable magnetic material to said rotor core exterior surface andan outer layer of high strength filament binding said outer layer ofremagnetizable magnetic material to said inner layer of remagnetizablemagnetic material.
 12. The method according to claim 11, wherein saidinner layer of high strength filament comprises magnetic high strengthwire.
 13. The method according to claim 12, wherein said inner layer ofhigh strength filament comprises high strength fibers.
 14. The methodaccording to claim 11, wherein said outer layer of high strengthfilament comprises non-magnetic wire.
 15. The method according to claim11, wherein said inner layer of high strength filament comprisesmagnetic high strength wire and said outer layer of high strengthfilament comprises non-magnetic wire.
 16. A method of constructing arotor for use in a high speed controlled-pole electric machinecomprising the steps of: positioning a plurality of magnetic blocksabout a rotor core exterior surface, said magnetic blocks forming aninner layer of remagnetizable magnetic material; and, binding said innerlayer of remagnetizable magnetic material to said rotor core exteriorsurface using an inner layer of high strength wire.